1. Field of the Invention
The present invention relates to a solid-state imaging device, an imaging apparatus, a pixel driving voltage adjustment apparatus, and a pixel driving voltage adjustment method. In particular, the present invention relates to a scheme for setting a transfer driving voltage level to a suitable value in a solid-state imaging device which sequentially performs charge transfer at a plurality of different transfer driving voltage levels.
2. Description of the Related Art
In various fields, a semiconductor device (in particular, called a solid-state imaging device) is used which detects signal charges by using a charge generator (a so-called sensor unit, such as a photodiode or the like), which is sensitive to electromagnetic waves from the outside, such as light or radioactive rays, and acquires an image on the basis of an electrical signal (pixel signal) according to the amount of the detected signal charges.
For example, in the field of video equipment, such as video cameras or digital still cameras, a CCD (Charge Coupled Device) type or an MOS (Metal Oxide Semiconductor) or CMOS (Complementary Metal-oxide Semiconductor) type solid-state imaging device is used which detects light (an example of electromagnetic waves) from among the physical quantities.
The solid-state imaging devices includes an amplification type solid-state imaging device in which a pixel signal generator, which generates a pixel signal according to the signal charges generated by the charge generator, is structured with pixels of an amplification type solid-state imaging device (APS; Active Pixel Sensor, also called a gain cell), which has an amplification driving transistor. For example, many CMOS type solid-state imaging devices are structured as such.
This amplification type solid-state imaging device is structured such that, in order to read out the pixel signal to the outside, address control is performed on a pixel portion having arranged a plurality of unit pixels, and a signal from each unit pixel is selected and read. That is, the amplification type solid-state imaging device is an example of an address control type solid-state imaging device.
A solid-state imaging device is used under various environmental conditions, so the input level of electromagnetic waves to the charge generator varies over a wide range. For example, in the case of outdoor photographing during daylight hours, the solid-state imaging device is used with an extremely large amount of incident light. In this case, a satisfactory image without saturation of a subject on the high luminance side is desired. Meanwhile, in the case of outdoor photographing during nighttime hours, the solid-state imaging device is used with an extremely small amount of incident light. In this case, an image with a satisfactory S/N ratio without a subject on the low luminance side not being buried with noise is desired. When a relatively dark subject is imaged under the condition that a high-luminance subject is present in the background, for example, when a person near a window is photographed from indoors, that is, in the case of a photographing scene with high contrast in which a bright subject and a dark subject are mixed, an image with a wide dynamic range from the person on the low luminance side to the background color of the window on the high luminance side is desired.
In order to obtain an image with a wide dynamic range, there is a need to set a long charge accumulation time with respect to a pixel with a low input level of electromagnetic waves, thereby realizing a high S/N ratio, and to avoid saturation with respect to a pixel with a high input level of electromagnetic waves. As the scheme to meet such a need, for example, there are the schemes described in JP-A-2001-189893 and JP-A-2007-151069. In all cases, a voltage (called an intermediate voltage) which does not reach a normal complete transfer level is used as a control voltage of a charge transfer unit (transfer gate, transfer transistor, and a readout selection transistor) which reads out the signal charges of a charge generator, and the readout of the signal charges is performed multiple times.
With these schemes, intermediate transfer is performed to drive the charge transfer unit with an intermediate voltage, and then complete transfer is performed to drive the charge transfer unit with a normal voltage. When the input level of electromagnetic waves is low, the signal charges generated by the charge generator for a predetermined period are not discarded to the pixel signal generator by the intermediate transfer, but are completely transferred together with the signal charges further generated by the charge generator for a subsequent period. Thus, a long charge accumulation time is set, so a high S/N ratio is realized. When the input level of electromagnetic waves is high, a part of the signal charges generated by the charge generator for a predetermined period are discarded to the pixel signal generator by the intermediate transfer so as to limit the saturation of the charge generator, and the combined components of the signal charges generated by the charge generator for a subsequent period and the remaining signal charge which are not discarded by the intermediate transfer are completely transferred.
While depending on the circuit structure or driving timing of the pixel signal generator, the pixel signal by the intermediate transfer and the pixel signal by the complete transfer maybe separately read out. Alternatively, when the pixel circuit is structured such that a charge accumulator is provided on the pixel signal generator side, the pixel signal may be read out in a state where the signal charges by the intermediate transfer and the signal charges by the complete transfer are added. In the former case, the pixel signal acquired by the intermediate transfer and the pixel signal acquired by the complete transfer are added, so a final pixel signal is acquired.